1. Field of the Invention
The invention relates to a method for dynamically absorbing shocks in a power shaft, in particular a supercritical shaft, as well as to a shock-absorbing architecture capable of implementing such a method.
2. Description of the Related Art
Power rotating shafts, in particular in turboshaft engines, have nominal operating ranges which can exceed their first critical speed of deflection. By definition, the operating range of supercritical shafts always exceeds their first critical speed. At resonance, which occurs when crossing a critical speed, power shafts undergo overload phenomena which amplify deformations and strain caused by the imbalances of the shaft.
A modal analysis of the architecture of a given power shaft—which typically has a front bearing and a rear bearing—makes it possible to calculate the values of the critical speeds, the shape of the modal deformations as well as the distribution of the strain energy between the components parts of the drive shaft: front, rear bearings and shaft linking these bearings.
An exemplary modal analysis of a given supercritical shaft supplies a value of the first critical speed equal to 15 000 rpm, that is 70% of its nominal speed, with a distribution of strain energy equal to 10% at the front bearing, 30% at the rear bearing and 60% at the shaft.
In order to absorb the amplified stress, which is associated with the use of supercritical shafts notably, shock-absorbing bearings with an oil film, also called “squeeze film”, make it possible to limit the amplitude of the overload caused when crossing the critical speed.
However, these shafts may have power tooth systems, which is generally the case on turboshaft engines with speed reducer used in the aeronautical domain when the rotation speed of the power shaft is high. The speed reducer makes it possible to convert the power in order to feed the receivers (helicopter main transmission gearbox, electric generator, etc.). In that case, the use of squeeze films with the bearings flanking the tooth systems of the power shaft is excluded. Because these bearings must have enough rigidity to limit any radial displacement of the pinion undergoing the meshing forces, in order to transmit the driving torque and avoid any disengagement or premature wear. Now, squeeze films require the radial displacement of the rolling-element system so that they can be compressed and produce their shock absorbing effect. The use of squeeze films is thus incompatible with these bearings used for flanking tooth systems.
The strain energy remaining always important at the rear bearing—where it is superior to 10%—and the shock absorption being difficult to achieve at the front bearing because of the presence of the power tooth systems, the shock absorbing systems of the architectures of engines with speed reducer and critical shaft were thus arranged at the rear bearings of the drive shaft.
However, in some modern architectures, the rear bearings do not participate any more in the modal deformation, no external shock absorption being then possible at these rear bearings. So, the front bearings concentrate, typically, about 25% of the strain energy and the shaft approximately 75%: the rear bearings—which do not operate as shock absorbers—absorb then practically no deformation (less than 1%). The bearings of the drive pinion become thus the only area where it is possible to provide the whole drive shaft with external shock absorption, 75% of strain energy in the shaft being an unacceptable percentage.